The present invention relates to a projector and, more particularly, to a projector which is devised to have a shorter optical path from the light valve to the screen by applying the combination of an elliptical mirror and a parabolic mirror for projecting, or a combination of two paraboic mirrors for projecting or a combination of a parabolic mirror and an elliptical mirror for projecting.
A conventional projector is composed of a light source, a liquid-crystal type light-valve (with a single liquid-crystal panel or three liquid-crystal panels to include a spectroscopic system), a projection lens and plane mirrors (single or two piece), and a combination unit comprising of a Fresnel lens and a lenticular one that is, a lens and screen. They work on the same well-known principle as a cathode ray tube (CRT) works. It is well known that the size of the depth of the projector's casing, accommodating therein an optical path from the liquid-crystal type light valve to the screen, mainly depends upon the size of the projection mirror (plane mirror) placed immediately before the screen. Many attempts have been made to produce a projector having a shorter optical path.
A projector is disclosed in the Japanese laying-open Utility Model No. 63-73792. This is a liquid display which includes a casing with a display window provided with a front transparent screen and which includes therein a transparent liquid-crystal display panel, a light source for illuminating the liquid-crystal display panel from the back and a projection lens for projecting an enlarged image of the liquid-crystal display panel onto the screen. In the casing, a prism having a light-emitting plane larger than the display window is placed with its light-emitting surface in contact with or near the internal surface of the display window and with its incident plane opposed to the projection lens directly or through the mirrors indirectry. The liquid-crystal display panel is placed opposite the projection lens directly or indirectly through the mirror.
A projector is disclosed in the Japanese Utility Model publication No. 4-38346, wherein rays of light images from a liquid-crystal display panel reflected by the plane mirror, pass through the projection lens and are reflected by the plane mirrors to form an image on the screen.
A projector is disclosed wherein rays of light emitted from the liquid-crystal panel through the projection lens are collected by a Fresnel lens to form an image on the lenticular screen. It is high-lighted by placing the liquid-crystal panel in a tilted state.
A projector is disclosed in the Japanese laid Patent No. 2-3556, wherein rays of light emitted from the light valve and disposed in a casing are reflected at the total reflection mirrors which are disposed parallel to each other in the casing and form an image on the screen composed of a Fresnel lens and a lenticular one. This lenticular screen is of a flap type in order to reduce the size of the depth of the projector while it is out of use.
The Japanese laid open Patent No. 2-157734 describes a conventional projection device that is composed of a light source, reflecting mirrors No. 1, 2 and 3, a color-active, liquid-crystal display and a screen with the purpose of attaining a reduced light loss in its optical system, a shortened optical path to a screen and a simple construction. The first reflecting mirror reflects incident light from the light source by its curved surface to produce a parallel light beam falling onto the color-active liquid-crystal display that transmits or shuts off the incident light for a pixel. The second reflecting mirror reflects a parallel beam of incident light transmitted through the color-active liquid-crystal display causing the light to enter into a third reflecting mirror which in turn reflects the incident light from the second reflecting mirror to project an image onto a screen.
As mentioned above, any conventional projector has an insufficiently shortened optical path: its optical system, therefore, is merely saved in depth size when being mounted on the floor or a wall.
For instance, the projectors have the depth sizes reduced to some extent by tightly placing the mirrors and the screen therein. However, they are not so small as to be called thin types.
The projector employs a total reflection-type prism which may involve leakage of external light causing considerable deterioration of the image to be formed. The application of the prism may also increase the projector's weight.
Both the projectors use a plurality of mirrors around a screen, aiming at reducing the size of a reflecting mirror before the screen. However, it is not effective to reduce the depth size of the projector. Moreover, multi-reflection may considerably effect the projected image's brightness.